I'll start off by saying that Father Christmas is called many names and operates in many different ways in different countries and even within different branches of the same families... - he is magical, but HOW he is magical is different so that makes it easier to decide how YOUR FAMILY want to approach it.
On your questions:
1. What does he bring? One present? Everything? Just the stocking?
It depends.
In our family, he brings some presents (sometimes 1 big present, sometimes it is a few smaller ones) and a stocking (loosely based on "the poem" but plenty in it - I'll get to that later). But there is always a present under the tree from DH and I for DD as well. Because there are presents under the tree from us to others, and from others to all 3 of us.
The only presents from Santa are those which DH and I, as his delegated Elves, get.
But I have seen others where ALL the presents for the DCs come from Santa, and only adults have presents under the tree (or adults get no presents in some cases). Or there are presents from others under the tree but none from Mum and Dad to the DCs because everything they bought (as Elves) is from Santa.
2. If he brings a stocking, what is in it? How many items go in a stocking?
See, I knew there'd be this question! 
Poem: Something you want, something you need, something to eat and something to read.
Basically, a mix of fun stuff and some practical (but nice versions of) things too.
Our "eat" is always some perfect fruit and some nice sweets, sometimes there have been interesting portion packs of biscuits or cereal from overseas etc.
Read - always at least 1 book, fiction, but sometimes a non-fiction book on an interest DC has or a 2nd fiction book might make its way there too.
Want - any kinds of fun stuff that we think DC would enjoy
Need - nice hair bobbins, character underwear, fancy socks, nice colouring pencils, a good pen.....useful things but nice versions that might not normally be bought. Could include travel sized toiletries as DC get older, or a gift card for places like McDs/Starbucks to go with friends....plectrums for a guitar player, tennis balls for a tennis player, paintbrushes or small sketch pad for someone who likes art....
There's often an overlap between the categories.
I do try and avoid cheap plastic things (pocket money items) although I have bought firecracker snaps before. But by keeping an eye out all year, I often find things like wooden toys, nice marbles, mini science kits or slinky's, things to add to existing collections, etc, and I have made a few things over the years as well.
The stocking is not huge (in real size, about 14" long and 8" wide, ish) so it is full enough to be bulky, usually with something spilling out onto the floor beside it, but not bags and bags of things.
3. How does he deliver the presents? Does he go round in a sleigh climbing through chimneys? Is it the elves? Do they appear by magic?
Yes, drives a sleigh pulled by 8 reindeer (Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donder, Blitzen) and Rudolph at the front with his red nose pointing the way. (Mix up between Twas the Night Before Christmas book and Rudolph the Red Nosed Reindeer, which have both entered common lore). Lots of magic involved.
The Santa Clause, a film starring Tim Allen, explains it well!
4. What is this magic everyone seems to be talking about anyway? Can Santa do magic? Do the elves do magic?
Yes, see above.
5. Do all the presents get manufactured by his elves or does he buy them directly from toy manufacturers?
It's a combination. Most films refer to the Elves workshops making everything (and wrapping everything in some cases). But there are also sooooo many DCs around the world that most parents know the Elves can't make everything for all of them. So sometimes some Elves go out and buy some things from other toy factories, like you see in shops. (Some Elves are very good at negotiations with factories and get everything there for the DCs on their lists, while other really lucky Elves have good relationships with some toymaker Elves and can get some things from the workshop).
6. What happens in a grotto?
Usually, Santa gets to talk to the DCs who come to visit, let him know what they might want from him, and maybe an Elf might take a photo or give them a present.
Some grottos have things for DCs to do with Santa or his helpers (baking with Mrs Claus, seeing Santa's hedgehog friend having a winter sleep and planting a tree, colouring a picture or painting a decoration....) but usually these are not in shops but in places like NT houses, or Winter Wonderland type venues.
7. Is the Santa in the grotto the real Santa?
Not normally, no, because there are lots of LOTS of grottos - have you not realised you see one at the supermarket and then he's in another shop, and then he's at the school fair, and how can he be everywhere?!
So there are lots of helpers who work in the Grottos, and give the real Santa lots of feedback, so all the messages the DCs give to every Santa get to the North Pole.
And the REAL Santa does go to some Grottos to meet some DCs himself, so you never know if you are talking to a helper Santa or the Real Santa!!
8. Where did Santa come from and how old is he?
Lots of theories on this one. But the Saint Nicholas story is a lovely one about giving gifts. There are people who think Santa is something like the Queen, and his child takes over at some point when the current Santa gets too old - Arthur Christmas is a good movie showing this dynasty idea. But he is certainly very very old. Have you not seen his snowy white beard!!
9. Is it generally accepted that Mrs Claus exists or is that something from American movies?
Absolutely, she looks after Santa and is a great source of fun (and of baking!) at the North Pole. (Lots of Elves are good bakers too!). And welcomes Santa home again at the end of his long annual flight.
10. How many elves are there? Are there any special elves?
Hundreds. Probably some. Films differ, but a nice place to learn about Elves is the Santa Update website, which explains all about Elves (and the whole North Pole operation) and also has a Santa Tracker running on Christmas Eve (slightly out of sync with the NORAD version - but both are great fun!).
11. Do the elves venture out of the North pole? Do they do anything else apart from on a shelf and manufacture toys?
Well they have to mind the reindeer, and someone has to cook for the other Elves and Santa, and make (or repair) the sleigh, and keep the North Pole neat and tidy.....and negotiate with the toy factories as well. Some also help to keep an eye on all the DCs around the world, and to compile the lists and answer (some of) the letters sent to Santa.
12. What does Santa like to eat and drink? I've seen various things from milk to whiskey. Personally I wouldn't mind if he liked Bailey's hot chocolate.
In our house, he likes milk (we're afraid of drink-flying). Growing up, he used to like Guinness. I know in another house, he always drains the flask of hot chocolate that the DCs leave out for him. I suspect he likes the variety though, I mean, imagine drinking 2 billion glasses of milk in 1 night!
The same goes for food - DC makes cookies every year (I keep a half batch of dough in the freezer to just slice and bake, just in case we don't have time to do them properly!), but lots of families will leave out a mince pie, slice of cake, biscuits from a packet - whatever they think he'd like.
Lots of families also leave out a carrot or stick of celery or an apple for the Reindeer too.
13. How does he enough time to visit all the children in the world? Is it timetravel? Is it magic? Is it a combination?
Combination. Again, Santa Update website is good on this, or The Santa Clause movie. And Christmas Eve night lasts a long time, as there is time differences to take into account across the globe! Some DCs are waking up with their presents already on Christmas morning (Japan, Australia etc) before Santa has even finished delivering in places like USA and Brazil.
14. Does he live in Lapland or the North pole?
Technically, he lives at the North Pole, but that used to be very near to Lapland years ago (magnetic variation means the actual North Pole moves around a bit), so you can use either or both.
15. Does he use technology or is he low tech?
I think, while he likes low tech, he does actually use technology to help him out. He still uses the reindeer to fly, and likes real letters, but has mechanised lots of manufacturing at the toy workshop (but there are still Elves doing things by hand). And he probably does use some of the protections in the Santa Clause and Arthur Christmas really - like navigation and something to protect against scorching in some houses where people forget to not light the fire on 24th!
16. Who delivers Christmas Eve boxes? Are they a generally accepted part of Christmas now and does everyone do them?
OOOh, that's another controversial question with no right or wrong answers.
In our house, Santa has never had an involvement in the Christmas Eve box, it has always been MY job. And partly explains why many things are re-used every year (Twas the night before Christmas book, plastic plate and glass with Santa on it from when DC was a toddler, Snowman-covered hot water bottle etc) as well as new things I would know we need like PJs and some nice hot chocolate and a Christmassy bath bomb.
But in some houses, Elves leave the box early in the day or late in the evening. The evening ones are usually similar to ours, but the earlier ones sometimes have things like a Christmassy craft, a DVD to watch, or tickets to a Panto, inside them.
And there are lots and lots of houses that don't have a CEB at all.
I hope that helps you a little! Enjoy figuring it out for YOUR family.
From BiddyPop, a certified Christmas nut!!